In the art of recovering oil from subterranean reservoirs, it is known that substantial volumes of oil are left in the reservoir after all effective primary production techniques have been utilized. Consequently, it has been necessary to resort to secondary recovery techniques which involve driving the oil from the reservoir with a fluid such as water, high and low pressure gases, such as natural gas, air carbon dioxide, etc., a small slug of a fluid such as propane, a surfactant solution and the like, which is mutually miscible with the reservoir oil, followed by a driving fluid, such as natural gas, in the case of a propane slug and, preferably, water, in the case of a surfactant slug, and various combinations of these driving media. Techniques utilizing a slug of surfactant solution have become of increasing importance, and interest in using the same has increased, since such techniques can be utilized in reservoirs which have already been subjected to a secondary recovery technique, particularly where the reservoir has been produced to its economic limits by water flooding. These latter techniques are commonly referred to in the petroleum industry as tertiary oil recovery techniques.
The primary advantage of the use of surfactants in oil recovery techniques is that the surfactant reduces the interfacial tension between the oil and the water to such an extent that substantially increased quantities of oil can be displaced. These surfactants have been used in both systems forming microemulsions and those not forming microemulsions. When a microemulsion is utilized to accomplish a miscible displacement of the oil, certain drawbacks result, including the difficulty of maintaining miscible displacement throughout the reservoir and the difficulty of maintaining the low interfacial tensions necessary to provide effective immiscible displacement after miscible displacement has broken down.
In recent years it has been suggested that microemulsions be utilized in a technique in which the microemulsion is immiscible with the oil and water in the reservoir. In these conventional techniques a multiphase microemulsion system is formed above ground by mixing oil, brine and surfactant and injecting at least the immiscible microemulsion phase. However, this technique requires that substantial amounts of oil be reinjected into the reservoir. Thus the cost of the operation is substantially increased and, to the extent an oil other than the reservoir oil is utilized, problems occur due to the differing phase behaviors of different oils.
In order to overcome the above-mentioned and other difficulties encountered in the prior art use of surfactants in oil recovery, it is proposed, in U.S. Pat. No. 4,079,785 by James E. Hessert, David F. Boneau and Richard L. Clampitt, issued Mar. 21, 1978 and application Ser. No. 804,132 filed by Gilbert R. Glinsmann on June 6, 1977, which are incorporated herein by reference, that an effective immiscible surfactant drive can be carried out by injecting a slug of surfactant solution comprising a surfactant, an electrolyte, water and, optionally, a cosurfactant to form a multiphase system in situ in the reservoir, which comprises; at least two different regions, for example, an oil-rich region and a microemulsion region. The latter application points out that best results are obtained when three different multiphase regions are formed, namely, a microemulsion in equilibrium with an oil phase (hereinafter referred to as a gamma-type region), a microemulsion in equilibrium with both an oil phase and a water phase (hereinafter referred to as a beta-type region) and a microemulsion in equilibrium with a water phase (hereinafter referred to as an alpha-type region). It is also pointed out in copending application Ser. No. 804,132 that among the variables which affect the three-phase region in which a particular system will partition are salinity, oil type, surfactant average equivalent weight, cosurfactant type, and temperature. Application Ser. No. 804,132 also goes on to point out that if all variables are fixed except the salinity, the system will shift from a gamma-type to a beta type to an alpha-type as the salinity increases from zero. Finally, application Ser. No. 804,132 sets forth a simple procedure, which can be carried out in a laboratory, to establish the system of water, electrolyte, surfactant and, optionally, cosurfactant and the proportions thereof which will be most effective for enhancing oil recovery when injected into the reservoir of interest.
Due to their availability and because of economic reasons, the most commonly utilized surfactants are petroleum sulfonates. In selecting an appropriate petroleum sulfonate for use in the recovery technique of the said application Ser. No. 804,132 it is highly desirable that one know the average equivalent weight of the petroleum sulfonate. While the average equivalent weights of a number of commercially available petroleum sulfonates have been established, this not the case with all commercially available materials and, in addition, it is often necessary to tailor the petroleum sulfonate for use in a particular oil recovery process, since the particular petroleum sulfonate found most effective may not be commercially available. In addition, as will be pointed out hereinafter, certain petroleum sulfonates do not act as expected with respect to their phase behavior.
Characterizing petroleum sulfonates by the average equivalent weight method described in ASTM Procedure D-855-56 is useable only for a sodium sulfonate and is very time consuming. Analytical characterization methods, based on anionic surfactant dye complexex, are also subject to various problems due to impurities in the dye, salt effects and the interference of unreacted oil in the sulfonate.
In light of the above it would be highly desirable to provide a fast, reproducible and accurate technique for determining the effective average equivalent weight of petroleum sulfonates.
It is therefore an object of the present invention to provide a fast, accurate and reproducible technique for characterizing petroleum sulfonates.
Another object of the present invention is to provide a fast, accurate and reproducible technique for characterizing petroleum sulfonates for use in the displacement of oil from a subterranean reservoir.
Another and further object of the present invention is to provide a fast, accurate and reproducible technique for determining the effective average equivalent weights of petroleum sulfonates.
Another object of the present invention is to provide an effective, fast and accurate technique for determining the effective average equivalent weights of petroleum sulfonates which will permit one to produce petroleum sulfonates tailored to be most effective in oil recovery processes.
These and other objects and advantages of the present invention will be apparent from the following description.